Hydroxyl groups that are in the β-position relative to a sulfur atom in an aliphatic chain have unusually high reactivity, and their properties are significantly different from other hydroxyl groups. For example, unlike compounds with hydroxyl groups in other positions, compounds with hydroxyl groups in the β-position relative to a sulfur atom in an aliphatic chain readily undergo self-polycondensation as well as co-condensation with other glycols in the presence of other acids and/or at elevated temperatures, resulting in the formation of poly(thioethers) (F. Richter, et. al., U.S. Pat. No. 2,582,605).
Di(hydroxyethyl)disulfide, as well as other di(hydroxyethyl)polysulfides are typical compounds with hydroxyl groups in the β-position relative to a sulfur atom. They are known in the art to be precursors for various poly(thioethers), which have been used in lubricants (U.S. Pat. No. 2,582,605), in polyurethanes (U.S. Pat. No. 3,386,963), in mercaptan-terminated oligomers (U.S. Pat. No. 4,124,645), in transmission fluids (U.S. Pat. No. 4,764,299), and in acetal-functional compounds used in window insulation (U.S. Pat. No. 6,383,324).
The prior art describes several attempts to convert di(hydroxyethyl)polysulfides into various compounds that contain ester functionality adjacent to the —(CH2)n—S— segment. For example, U.S. Pat. No. 6,114,485 discloses compounds that include an —O—C(O)—(CH2)2—S— segment in monomeric products, but the chemical structure of these compounds is achieved through a chain of several complex chemical reactions that take multiple steps and over 20 hours of combined reaction time. In addition, the technology described in this patent cannot be used to produce polymeric products with multiple poly(thioester) segments.
U.S. Pat. Nos. 2,221,418 by Weihe et al. (referred to hereafter as Weihe) and 5,407,972 by Smith et al. (referred to hereafter as Smith) describe products that are produced after (polythio)glycols are mixed with dicarbonic acids and/or their anhydrides. However, these patents do not describe the formation of poly(thioesters) from these products. For example, Weihe describes the formation of an “insoluble balsam”, and Smith describes “polysulfide polymers” produced as the result of the interaction between di(hydroxyethyl)polysulfides and dibasic carbonic acids or their anhydrides.
Nowhere in Weihe or Smith is described the chemical structure of the resulting products. However, based on the above-described unusual reactivity of hydroxyl groups in the β-position relative to a sulfur atom, and the strong tendency of such hydroxyl groups to homo-condense according to reaction (1), it is highly likely that the products formed by Weihe and Smith under the conditions described in these patents are poly(thioethers), rather than poly(thioesters).mHO—(CH2)2—Sx—(CH2)2—OH→H(—O—(CH2)2—Sx—(CH2)2)m—OH+(m−1)H2O  (1)In the case where the products were formed with the participation of dibasic carbonic acids, they would likely form a solution of dibasic carbonic acids in solid or semi-solid poly(thioether) resins. In the case where the products were formed with the participation of anhydrides of dibasic carbonic acids, the solid or semi-solid poly(thioether) resins would have a chance to react with anhydrides. This would allow the formation of a randomly-formed compound with no more than two radicals per molecule and a single ester structure for each radical. A regular poly(thioester)polymer would not be formed.
The absence in the prior art of the description of regular poly(thioesters) produced from compounds with hydroxyl groups in the β-position relative to a sulfur atom is further illustrated by Wilson in U.S. Pat. No. 5,342,724 (referred to hereafter as Wilson). Wilson describes the formation of multiple poly(thioesters) from sulfur-containing diols and dibasic carbonic acids. However, all sulfur-containing diols with hydroxyl groups in the β-position relative to the sulfur atoms were left out from the list of diols mentioned by Wilson, as the state-of-the art technology available at the time did not allow production of poly(thioesters) from such compounds.
Accordingly, there is a need in the art to develop methods of forming poly(thioesters) from sulfur-containing diols with hydroxyl groups in the β-position relative to the sulfur atoms.